Evolution of Low Noise Receiver Design for Ka-Band ... · The LNB for Ka-band satellite terminals...

American Institute of Aeronautics and Astronautics

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Evolution of Low Noise Receiver Design

for Ka-Band Satellite Terminals in Hostile Environments

Vesa Waris 1

EM Solutions, Brisbane, Qld, Australia 4105

Low Noise Block Converters (LNBs) are the principal components used in receivers for

the frequency translation of incoming signals from a satellite to a modem. With the

widespread use of Ka-band satellite terminals in a range of environments, and particularly

for SATCOM on the Move (SOTM) and airbourne and maritime applications, the

ruggedisation requirements for the hardware have increased. This paper describes the

evolution of the LNB from a single module to a split design for SOTM, and discusses some of

the issues that have arisen to achieve reliable operation in a range of increasingly hostile

environments.

I. Introduction

O achieve reliable operation from a satellite ground terminal, sufficient RF performance is required from the

Block Upconverter (BUC) and the Low Noise Block Converters (LNB). The LNB translates the received signal

from the satellite to a suitable frequency for a modem. Typically, a military Ka-band satellite operates with a down-

link in the 20.2-21.2GHz range. This 1GHz band is translated to L-band, typically 1-2GHz or 950MHz to 1.95GHz,

which is applied to the receive port of a satellite modem. The LNB performs a critical function for the receiver of a

satellite terminal - to achieve sufficient carrier to noise performance - and is thus required to have a very low noise

figure, typically 1.3dB, and high gain, typically 60dB. Additionally the LNB is required to have good image

rejection and filtering of the upconverted signals at the same terminal.1

The LNB for Ka-band satellite terminals consists of a WR42 waveguide input, followed by a low noise amplifier

(LNA), an image reject filter, and frequency conversion to the L-band interface, as shown in the block diagram of

Fig.1. A local oscillator (LO) with very low phase noise and high immunity to vibration is used for translating the

satellite receive band to the L-band frequencies.

Figure 1. Block diagram of Ka-band LNB.

Apart from the key RF parameters, the LNB is required to provide reliable operation in a wide range of

environments and over a wide temperature range. Typical satellite terminals are fixed or transportable outdoor

systems with a parabolic dish antenna and with the LNB mounted close to the feed. Some LNBs for Ka-band

satellites are used in SATCOM on the Move (SOTM, also known as Communications on the Move - COTM) in

land-based, maritime or airbourne platforms. These applications present greater challenges in terms of the operating

environment. This paper will describe some of the challenges faced in developing the LNB to operate from a

relatively benign, fixed ground-based environment to more demanding moving platforms and hostile environments.

1 Senior RF Engineer, EM Solutions Pty Ltd, 55 Curzon St, Tennyson Qld, AUSTRALIA.

T

ICSSC 7-10 September 2015, Gold Coast, QLD, Australia 33rd AIAA International Communications Satellite Systems Conference AIAA 2015-4338

Copyright © 2015 by EM Solutions Pty Ltd. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


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II. Fixed Ground-based LNBs

Figure 2 shows a photograph with two views of a Ka-Band LNB. The approximate dimensions are 100mm (L) x

65mm (W) x 51mm (H). The LNB has a WR42 waveguide receive port which connects to the antenna feed. The

LNB shown also has a single SMA connector for the downconverted L-band output, as well as being used to provide

DC bias to power the module, and conducting the reference signal for the on-board local oscillator used for the

frequency translation. Most LNBs will be a variation of this, with some having different RF connectors or separate

multi-pin connectors for DC and for monitoring. Mounting arrangements also vary, depending on the requirements

of the installation. The LNBs shown in Fig.2 have mounting feet for installation on a platform, but in many cases,

the low mass (400g typ.) has enabled the LNB to be mounted directly to the waveguide structure of the antenna feed

with little or no additional support. The relatively low DC power consumption means that fans are generally not

required for the LNB.

Figure 2. Photograph of Ka-band LNB showing front and rear views.

The LNBs are historically made as a sealed unit, to withstand environmental conditions to IP65 or greater.1,2

This has included the use of O-rings for sealing all of the joints, screws and connectors, as well as external painting.

A waveguide pressure window has ensured that the waveguide port is also sealed. The LNBs may also be

pressurised as a further measure to prevent water ingress. These design factors have ensured that the LNBs have

provided many years of reliable operation under the environmental conditions faced in typical outdoor fixed

installations.

III. Airbourne LNBs

Airbourne applications for LNBs has resulted in additional requirements to be met. The LNB is required to meet

standards from DO-160 for approved operation on airbourne platforms.3 The standard includes environmental

conditions, vibration and shock, as well as EMI/EMC and lightning susceptibility. In terms of the environmental

specifications, the operating altitude and temperature ranges have increased from those of the standard LNB. The

minimum operating temperature has gone from -20°C or -40°C for standard LNBs to -55°C, while the maximum

operating temperature has increased from 60°C to to over 70°C. This has required careful component selection and

some re-design from the standard LNB to ensure that the wider operating and storage temperatures are satisfied.

For the airbourne application, the LNB is also required to operate to altitudes well over 13000m. This has meant

that the airbourne LNB is no longer a sealed unit, as for the ground-based models. A breather hole with a moisture

patch has been included to allow the air pressure to equalise between the LNB internals and the surrounding space,

while still maintaining a barrier to water and dust ingress.

The DO-160 airbourne LNB requirements for lightning susceptibility have required additional protection

circuitry to ensure the reliability of the electronics under induced lightning effects. Figure 3 shows a test setup

where some direct lightning waveforms with multiple peak pulses of over 1000kV were applied to the chassis of an

LNB, with no deleterious effects observed.




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Figure 3. Photograph of Ka-band LNB undergoing lightning testing.

IV. SOTM LNBs

For SATCOM on the Move (SOTM) terminals manufactured by EM Solutions,4 the LNB is no longer a

component that is sold separately to a system integrator. The SOTM terminal is the system into which the LNB is

integrated. In the system architecture of the SOTM terminal, the function of the LNB was split between the LNA

and the low-noise downconverter, which were physically located apart from each other. While this was mainly due

to a requirement to provide an RF input to the monopulse tracking technology used in the system, a secondary

reason was to reduce the size and weight of the movable antenna feed, on which the LNAs were fixed. Figure 4

shows a photograph of a separate LNA sub-module that was used in a Ka-band SOTM system. The separate LNA

performs a critical function in the SOTM terminal, such that the overall system performance is highly dependent on

the LNA specifications.

Figure 4. Photograph of Ka-band LNB submodule.

In the standard LNBs previously described, the internal sub-modules are all well protected within the sealed

enclosure and isolated from the external environment. With the splitting of the LNB into separate LNA and

downconverter sub-modules, the previously used sealed enclosure is no longer present, potentially reducing the

environmental protection for the sub-modules. It is to be noted that the SOTM terminal has its own radome which

provides the bulk of the protection from the external environment, such as shown in Fig.5.




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Figure 5. Photograph of typical radome for SATCOM on the Move (SOTM) terminal.

It was discovered during environmental testing that, if the airconditioned radome was opened in a high humidity

environment, some moisture was able to condense on internal surfaces within the system. Following this, the

system could exhibit a reduced Carrier to Noise performance for the satellite downlink. After some investigation, it

was found that this may have been due to water droplets forming on various surfaces of the system. One of these

potential surfaces was within the LNA itself. Further experimentation showed that droplets of moisture in particular

parts of the circuit could lead to bias changes and a reduction in gain in the low noise HEMT amplifier. For

example, any condensation around one of the DC block capacitors between the HEMT stages may result in a

resistance of around 100k-300kΩ, as shown in Fig.6. Instead of blocking the DC between the drain voltage of one

stage and the gate voltage of the following stage, the condensation could disrupt the normal bias to the HEMT.

Since the gate resistance of the HEMT is very high (hundreds of kΩ), the any moisture could lead to the gate voltage

being pulled higher, potentially to 0V or slightly positive. This could result in an increase in the drain current and a

reduction in the drain voltage, lowering the RF gain of the amplifier and leading to a reduced Carrier to Noise

performance of the satellite receiver.

Figure 6. Partial schematic of Ka-band LNA showing potential resistive path due to moisture condensation.

To reduce the potential for moisture issues in high humidity environments, selected parts of the LNA PCB have

been conformally coated, so that the moisture condensation can no longer bridge the conductive surfaces and disrupt

the normal operation of the LNA. Improved sealing of the LNA cavity has also been implemented.

Another solution that is being explored to counter moisture effects is to change the RF lineup of the LNA. There

are, at present, no packaged MMIC amplifiers available that can meet the low noise figure requirements of the LNA.

However, there are a number of MMIC amplifiers that have a relatively low noise figure of around 2 to 2.5dB.

These amplifiers are potential candidates for a second-stage amplifier, with the low noise provided by a first stage

HEMT. The advantage of this approach is that separate DC block capacitors in the RF path are no longer required,

since they are internal to the MMIC amplifier. This means that the RF path should be more resilient in the presence

of moisture, since the drain voltage of the first stage can no longer affect the bias of the second stage amplifier. The

MMIC amplifiers are also typically sealed separately with a covering lid, which would provide these components

with additional protection from moisture.




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While the above solutions minimise the potential of moisture to affect the performance, any condensation

formed on the RF track could still result in a detuned response, since the dielectric constant of water is about εr~80.

To ensure that there is no moisture ingress requires a sealed LNA with O-rings or with hermetic sealing.

V. Maritime LNBs

The extension of the SATCOM on the Move (SOTM ) terminal to maritime platforms has also required

additional environmental protection. The satellite terminal is required to meet various parts of the maritime

standards of IEC-60945 and of IEC 60092.5,6

The external surfaces, particularly the covering radome, are required

to provide a higher level of protection from the more hostile maritime environment. However, to ensure reliable

operation in the maritime environment, the LNA and downconverter require sealed enclosures. This means that O-

rings are required for the lids, the waveguide and for the connectors. Waveguide windows for the LNA, as well as

other waveguide apertures are also required for prevention of moisture and salt spray ingress.

VI. Conclusions

The paper has described the evolution of a ground-based Ka-Band LNB to the harsher environments of

SATCOM on the Move (SOTM) terminals. In a stand-alone LNB, a sealed enclosure provides environmental

protection for the LNA and downconverter submodules, and has a proven reliability track record over many years of

operation in outdoor fixed installations. For airbourne LNBs, the more stringent temperature, altitude and other

environmental and electrical requirements have entailed some careful re-design from the standard LNB. For the

land-based SOTM terminals, some lessons were learned to ensure sufficient sealing and moisture retardation. These

have been applied to the even more stringent requirements of maritime SOTM terminals for Ka-band satellite

receivers.

References 1 Datasheet for Ka-Band LNB Part number 01-317B, EM Solutions Pty Ltd, URL:

http://www.emsolutions.com.au/downloads/datasheets/01-317B%20Wideband%20Ka%20LNB.pdf 2 International Standard, IEC 529, “Degrees of protection provided by enclosures (IP Code)”, Second edition, 1989-11,

International Electrotechnical Commission. 3 RTCA DO-160G, Environmental Conditions and Test Procedures for Airbourne Equipment, 2010, Radio Technical

Commission for Aeronautics. 4 Datasheet for Comms-On-The-Move, EM Solutions Pty Ltd, URL:

http://www.emsolutions.com.au/downloads/datasheets/COTM.pdf 5. IEC 60945 - Maritime Navigation And Radiocommunication Equipment And Systems - General Requirements – Methods

Of Testing And Required Test Results, Fourth edition, 2002-08, International Electrotechnical Commission. 6 IEC 60092-101 - Electrical Installations In Ships - Part 101: Definitions And General Requirements, 4.1 edition, 2002-08,

International Electrotechnical Commission.



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Evolution of Low Noise Receiver Design for Ka-Band ... · The LNB for Ka-band satellite terminals...

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